The present invention relates to inspection apparatus for a wiring pattern of devices such as a printed board and photomask.
In order to meet the recent requirement for size-reduction and weight-reduction, the precision and complication of a wiring pattern of a printed board or the like increasingly advance whereby difficulty is encountered to keep a high accuracy by a human-made inspection system. Thus, it is strongly desired to automatically and accurately perform the wiring pattern inspection. Various types of wiring pattern inspection systems have been devised hitherto. One known approach is a defect detecting system such as is disclosed in "Machine-Vision Techniques for Inspection of Printed Wiring Boards and Thin-Film Circuits", Optical Society of America, Vol.3, No.3/September, pp1465-1482, 1986, written by Jorge L. C. Sanz and Anil K. Jain. The conventional wiring pattern inspection techniques are generally classified mainly into the so-called design-rule system and comparison system. Of these conventional wiring pattern inspection techniques, there is a promising system arranged to detect defects of a wiring pattern by contracting or expanding bi-level image data to cause a wiring pattern defect before the thinning process, the system being disclosed by Jon R. Mandevile in "Novel Method for Analysis of Printed Circuit Images", IBM J.Res.DEVELOP., Vol.29, No.1, January, 1985. This conventional wiring pattern inspection system will briefly be described hereinbeow with reference to FIG. 1 where (a) to (d) are illustrations for detection of disconnection and (e) to (h) are illustrations of detection of short. In (a) of FIG. 1, let it be assumed that a point a represents a non-defective portion and points b and c respectively designate fatally defective portions accompanying the possibility of light-width abnormality or disconnection. In an operation step 1, a contraction process of the image is effected where the image is scraped or removed by one picture element from the circumference thereof so that the defect appears as disconnections as illustrated in (b) of FIG. 1. An operation step 2 is then performed so as to effect the thinning process where it is further scraped by one picture element from the circumference to become a line-like image as illustrated in (c) of FIG. 1. A subsequent operation step 3 is provided in order to perform scanning with a 3.times.3 logic mask to detect in accordance with a look-up table (LUT) as illustrated in (d) of FIG. 1. That is, with this process, the portions indicated by the points b and c are detectable to be disconnected. Further, the junction point between a terminal portion and the wiring pattern may be detected at this time.
In (e) of FIG. 1, let it be assumed that points b and respectively show fatally defective portions accompanying the possibility of the line-separation abnormality and the short. An operation step 1 is first executed so as to effect the expansion process where the image is expanded by one picture element from the circumference thereof so that the portion indicated by the point b enters into a shorted state as illustrated in (f) of FIG. 1. An operation step 2 follows to perform the thinning process to form lines as illustrated in (g) of FIG. 1. A subsequent operation step 3 is further executed so as to perform scanning with a 3.times.3 logic mask to detect defects in accordance with a look-up table (LUT) as illustrated in (h) of FIG. 1. That is, with this process, the portions indicated by the points b and c are detectable as the short. Here, the image process such as the thinning process and expansion process is known as exemplified by description in textbooks such as "Elements of Image Recognition [1]" published by Ohm Sha.
One of the major problems arising in such a wiring pattern inspection system is, however, that a number of processes including the image-contraction or image-expansion processes and thinning processes are required in the case of setting a plurality of minimum line widths and/or minimum line separations to thereby increase a load to the hardware. In addition, there is a problem in that difficulty is encountered in terms of ensuring accurate discrimination between the wire disconnection and the line-width abnormality and between the line-separation abnormality and the short. Further, according to this technique, difficulty is also encountered to distinguish between the T-branch and the short. Although the discrimination therebetween can be made in advance by learning the coordinate of the T-branch through a desirable printed board with no defect and comparing it with that of a printed board to be detected, this method also provides a problem that it is difficult to perform an accurate positioning between the desirable printed board and the printed board to be inspected. That is, in this case, it is required that the positioning accuracy is within .+-.0.3 to 0.6 mm.